1. Field of the Invention
The present invention relates generally to a handover apparatus and method in a mobile communication system, and in particular, to a handover apparatus and method using a relay node in a relay-based system.
2. Description of the Related Art
In general, communication systems have been developed based on voice service and are advancing to data service and various multimedia service beyond the voice service. The voice-oriented communication systems are not satisfying users' increasing service demands because of a relatively small transmission bandwidth and expensive fees. Additionally, the advance of the communication industry and the users' increasing demand for Internet service raise necessity for communication systems capable of efficiently providing Internet service. To respond to this, a broadband wireless access system has been developed to efficiently provide Internet service with the bandwith wide enough to meet the user's increasing demands.
The broadband wireless access system utilizes an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) scheme for a physical channel. In other words, using the OFDM/OFDMA scheme, the broadband wireless access system enables high-speed data transfer by transmitting a physical channel signal using a plurality of subcarriers. Hence, the broadband wireless access system is suggested as the strong candidate for the Fourth Generation (4G) wireless communication system.
Meanwhile, a cellular mobile communication system uses handover to provide seamless service to a terminal. When the traveling terminal migrates from an initial Base Station (BS) coverage (cell area) to an adjacent BS coverage, the connection established between the former BS and the terminal is handed over to a new BS.
Now, handover in an IEEE 802.16 system, which is one of the broadband wireless access systems, is described.
To support handover of the terminal, the BS broadcasts information relating to neighbor BSs using a Mobile Neighbor Advertisement (MOB_NBR_ADV) message. The MOB_NBR_ADV message includes a plurality of Information Elements (IEs) including a Frequency Assignment (FA) index, which is an identifier and a physical channel number of the individual neighbor BS. The terminal performs the scanning using the information of the MOB_NBR_ADV message broadcast by the BS and selects a target BS to be handed over according to the scanning result. Upon selecting the target BS, the terminal acquires the synchronization by receiving a preamble of the target node, acquires information required for the network reentry procedure by receiving control information messages (e.g., DownLink-Mobile Application Part (DL-MAP), UpLink-Mobile Application Part (UL-MAP), Downlink Channel Descriptor (DCD), Uplink Channel Descriptor (UCD), etc.) of the target node, and then performs normal communication after finishing the network reentry procedure.
However, in the handover, a terminal on the cell edge cannot normally receive information from the BS because of signal attenuation caused by the distance to the BS, i.e., the terminal on the cell edge needs to receive the information of the target BS as well as the information of the serving BS to execute the handover. However, the terminal cannot receive the control information message because of the distance to the BS. In the worst case, handover failure may cause service interruption.
As for the broadband wireless access system operating in a high frequency area, its transmitting rate and the service coverage are limited due to high path loss. To address those problems, a multi-hop signal transfer scheme has recently come under development. The multi-hop technique enables the high speed data communication with the reduced the path loss by relaying data using a Relay Node (RN), and the service coverage expansion by transmitting a signal to a Mobile Station (MS) far away from the BS.
Accordingly, using the relay technique, the RN can forward the control information message from the BS to the MS without loss. Currently, a handover optimization technology for optimizing time and resource required for the handover is under consideration. If the MS acquires the control information of the target BS required for the handover procedure in advance before the MS migrates to the target BS, the handover optimization can be achieved. In other words, when the RN is used for the handover, a new method for optimizing the handover is needed.